Cathodic electrodeposition coatings, their production and their use

ABSTRACT

Aqueous, lead-free cathodically depositable (CDL) coating compositions containing binders and, optionally, crosslinking agents, pigments, and/or additives conventionally employed in lacquers, as well as one or more sulfonic acid salts of the metals vanadium, manganese, iron, zinc, zirconium, silver, tin, lanthanum, cerium and/or bismuth and/or of organometallic compounds in a total amount of from 0.2 to 2 wt. %, calculated as metal and based on the resin solids.

The invention relates to cathodically depositable dipping lacquers(CDL), their production and their use in methods of coating electricallyconductive substrates by cathodic dipcoating.

A wide variety of CDL coating compositions are known. The patentliterature discloses many examples of ecologically advantageous,lead-free CDL coating compositions containing many different metalcompounds, especially tin and/or bismuth compounds, as crosslinkingcatalysts. For example, there are known from WO 93/24578 CDL coatingcompositions that contain bismuth salts of aliphatic hydroxycarboxylicacids as catalysts. WO 98/10024 describes CDL coating compositionscontaining catalytically active mixtures of bismuth and aminocarboxylicacids. EP-A-0 509 437 describes CDL coating compositions that containdialkyltin dicarboxylates derived from aromatic carboxylic acids ascrosslinking catalysts, as well as bismuth or zirconium compounds asfurther catalysts.

CDL baths are pumped round continuously and are subjected to shearstress. An important criterion for the quality of CDL coatingcompositions is their shear stability. Shear instability manifestsitself in the case of CDL coating compositions as sedimentation. Suchsedimentation leads in practice to the inclusion of dirt in the CDLlayer, especially during the process of applying CDL coatings tosubstrate surfaces in the horizontal position. Such sedimentationphenomena can be quantified in the laboratory by determining the sievingresidue of CDL baths.

The object of the invention is to provide lead-free cathodicallydepositable coating compositions having good shear stability.

It has been found that this object can be achieved with aqueouslead-free CDL coating compositions containing binders and, optionally,crosslinking agents, pigments, and/or additives conventionally employedin lacquers, which coating compositions are characterised in that theycontain sulfonic acid salts of vanadium, manganese, iron, zinc,zirconium, silver, tin, lanthanum, cerium and/or bismuth and/or sulfonicacid salts of organometallic compounds, preferably of tin, in a totalamount of from 0.2 to 2 wt. %, calculated as metal and based on theresin solids (based on binders, any crosslinking agents present and anyother resins contained in the CDL coating composition, such as, forexample, paste resins).

The lead-free CDL coating compositions according to the invention areaqueous electro-dipping lacquers known per se which can be deposited atthe cathode, to which there are added sulfonic acid salts of vanadium,manganese, iron, zinc, zirconium, silver, tin, lanthanum, cerium and/orbismuth and/or organometallic sulfonic acid salts, preferably of tin,for example, as crosslinking catalysts replacing lead catalysts.

The CDL coating compositions according to the invention are aqueouscoating compositions having a solids content of, for example, from 10 to30 wt. %. The solids content consists of the resin solids, of thecontent of sulfonic acid salts or organometallic sulfonic acid saltsthat is essential to the invention, any pigments and/or fillers andfurther additives that may be present. The resin solids consists ofconventional CDL binders, which carry cationic substituents orsubstituents that can be converted into cationic groups as well asgroups capable of chemical crosslinking, and any CDL paste resins andcrosslinking agents that may be present. The cationic groups may becationic groups or basic groups that can be converted into cationicgroups, for example, amino, ammonium, e.g., quarternary ammonium,phosphonium and/or sulfonium groups. Binders having basic groups arepreferred. Basic groups containing nitrogen, such as amino groups, areparticularly preferred. Such groups may be present in quaternary form orthey are converted into cationic groups in the manner known to theperson skilled in the art using a conventional neutralising agent,especially an inorganic or organic acid, such as, for example, asulfonic acid such as amidosulfonic acid (sulfamic acid) ormethanesulfonic acid, lactic acid, formic acid, acetic acid. The degreeof neutralisation is, for example, from 20 to 80%.

In addition to the sulfonic acid salts present according to theinvention, the CDL coating compositions may contain additivesconventionally employed in lacquers, for example conventionally employedin CDL coating compositions, Examples thereof are wetting agents,anticratering agents, flow agents, antifoams, as well as organicsolvents customarily used for CDL coating compositions. Examples of suchsolvents are alcohols, such as, for example, cyclohexanol,2-ethylhexanol; glycol ethers, such as, for example, methoxypropanol,ethoxypropanol, butoxyethanol, diethylene glycol diethyl ether; ketones,such as for example, methyl isobutyl ketone, methyl ethyl ketone,cyclohexanone; hydrocarbons.

The cationic or basic binders may be, for example, resins which containprimary, secondary and/or tertiary amino groups and the amine numbers ofwhich are, for example, from 20 to 250 mg KOH/g. The weight-averagemolar mass (Mw) of the CDL resins is preferably from 300 to 10,000. Theresins that may be used according to the invention are subject to nolimitation. It is possible to use the many different self-crosslinkingCDL binders and CDL binder/crosslinking agent combinations thatcrosslink by external means known from the extensive patent literature.Examples of such CDL resins are amino (meth)acrylate resins, aminoepoxyresins, aminoepoxy resins having terminal double bonds, aminoepoxyresins having primary OH groups, aminopolyurethane resins,amino-group-containing polybutadiene resins or modified epoxyresin-carbon dioxide-amine reaction products. Such binders may beself-crosslinking, or they are used in admixture with known crosslinkingagents familiar to the person skilled in the art. Examples of suchcrosslinking agents are aminoplastic resins, blocked polyisocyanates,crosslinking agents having terminal double bonds, polyepoxy compounds,crosslinking agents having cyclic carbonate groups, or crosslinkingagents that contain groups capable of transesterification and/ortransamidation.

In addition to the CDL binders and any crosslinking agent that may bepresent, and the content of sulfonic acid salts that is essential to theinvention, the CDL coating compositions according to the invention maycontain pigments, fillers, and/or additives conventionally employed inlacquers. Suitable pigments and/or fillers are the conventionalinorganic and/or organic pigments. Examples are carbon black, titaniumdioxide, iron oxide, kaolin, talcum or silicon dioxide, phthalocyaninepigments and quinacridone pigments, as well as anticorrosive pigments,such as zinc phosphate. The nature and amount of the pigments aredependent on the intended use of the CDL coating compositions. If clearcoatings are to be obtained, then no pigments or only transparentpigments, such as, for example, micronised titanium dioxide or silicondioxide, are used. If opacifying coatings are to be applied, then theCDL bath preferably contains colour-giving pigments.

The CDL coating compositions according to the invention contain sulfonicacid salts of vanadium, manganese, iron, zinc, zirconium, silver, tinlanthanum, cerium and/or bismuth and/or organometallic sulfonic acidsalts, preferably of the above metals and particularly preferably oftin. Sulfonic acid salts of bismuth and organotin sulfonic acid saltsare preferred. The sulfonic acid salts and organometallic sulfonic acidsalts, together also referred to hereinbelow as “sulfonic acid salts”for short, are salts derived from one or more mono- or poly-basic,preferably water-soluble sulfonic acids. Examples of sulfonic acid fromwhich the sulfonic acid salts contained in the CDL coating compositionsaccording to the invention may be derived are amidosulfonic acid and/ororganic sulfonic acids, such as N-alkylamidosulfonic acids, for exampleN-C₁-C₄-alkylamidosulfonic acids; alkanesulfonic acids, which may besubstituted in the alkyl radical, such as methanesulfonic acid,ethanesulfonic acid, propanesulfonic acid, trifluoromethanesulfonicacid, hydroxyethanesulfonic acid, hydroxypropanesulfonic acid;arylsulfonic acids, which may be substituted in the aryl radical, suchas benzenesulfonic acid, benzenedisulfonic acids, p-toluenesulfonicacid, 1- or 2-naphthalenesulfonic acid, dodecylbenzenesulfonic acid.Preference is given to the salts of monobasic sulfonic acids. Specialpreference is given to the salts of monobasic alkanesulfonic acids,especially salts of methanesulfonic acid.

The proportion of sulfonic acid salts in the CDL coating compositionaccording to the invention is from 0.2 to 2 wt. %, preferably from 0.5to 1.5 wt. %, calculated as metal and based on the resin solids in theCDL coating composition. The sulfonic acid salts or organometallicsulfonic acid salts may be present in the CDL coating compositionsaccording to the invention dissolved or finely distributed in theaqueous or in the disperse phase.

The metallic sulfonic acid salts or organometallic sulfonic acid saltscan be prepared by reacting the sulfonic acids with suitable metalcompounds, for example metallic oxides, hydroxides or carbonates, suchas, for example, preferably bismuth oxide, or with suitableorganometallic compounds, for example hydrocarbyl metallic oxides, suchas, for example, preferably dialkyltin oxides, such as, for example,dibutyltin oxide or dioctyltin oxide. It is possible to react one ormore metallic compounds in a mixture and/or one or more organometalliccompounds in a mixture with one or more sulfonic acids. The saltformation may be carried out, for example, in the presence of water,optionally in admixture with organic solvents.

Salt formation by reaction of the metallic or organometallic compoundswith the sulfonic acids can be carried out stoichiometrically or with adeficient amount or with an excess of sulfonic acid. This is explained,in the case of metallic sulfonic acid salts, using the example ofbismuth sulfonic acid salts, which are preferred, which can be formed,for example, by reaction of bismuth oxide (Bi₂O₃) with a monobasicsulfonic acid in a molar ratio of 1:2 to, for example, 8, preferably 1:3to 7, a molar ratio of bismuth oxide to monobasic sulfonic acid of 1:6corresponding to a stoichiometric reaction. The same applies toorganometallic sulfonic acid salts, and this is explained using theexample of dialkyltin sulfonic acid salts, which are preferred, whichcan be formed, for example, by reaction of dibutyltin oxide (Bu₂SnO)with a monobasic sulfonic acid in a molar ratio of 1:0.5 to, forexample, 3, preferably 1:1 to 2.5, a molar ratio of dibutyltin oxide tomonobasic sulfonic acid of 1:2 corresponding to a stoichiometricreaction. After the reaction, excess sulfonic acid can be separated fromthe sulfonic acid salt that has formed or can be introduced therewithinto the CDL coating composition according to the invention and serve asthe sole neutralising agent or as part of the acids altogether containedtherein as neutralising agent. The sulfonic acid salts can be isolatedas such and used in the production of the CDL coating compositionsaccording to the invention, or they are used, without being isolated andpurified, in admixture with further constituents of the reaction mediumfrom the preparation of the sulfonic acid salts, for example in the formof an aqueous solution.

The sulfonic acid salts can be incorporated into the CDL coatingcompositions in various ways. For example, the sulfonic acid salts maybe added to an optionally already neutralised CDL binder solution beforeaddition of substantial amounts of water as diluent, and then behomogenised with stirring. Alternatively, when the correspondingsulfonic acids are used as neutralising agent for the binders, it isalso possible, for example, to work with the metallic oxide, hydroxideor organometallic oxide, the corresponding sulfonic acid salts beingformed in situ; in that case there is advantageously used an increasedamount of sulfonic acid adapted according to the amount of sulfonic acidrequired for neutralisation of the CDL binders, in general correspondingto a degree of neutralisation of over 100%. It is preferred to add thesulfonic acid salts to the finished aqueous CDL binder dispersion or tothe finished CDL coating composition per se, for example in the form ofa sulfonic acid salt solution, e.g., in the form of an aqueous sulfonicacid salt solution. Addition in the form of a sulfonic acid saltsolution may also take place at an earlier stage in the production ofthe CDL coating compositions according to the invention.

Regardless of whether the CDL coating compositions according to theinvention are pigmented or transparent, they can be produced, forexample, entirely by the so-called single-component process. In the caseof pigmented CDL coating compositions, single-component concentrates areprepared by distributing, for example dispersing, and optionally millingpigments and fillers in the organic solution of a CDL binder, forexample of a paste resin suitable for CDL coating compositions, addingfurther organically dissolved CDL binder, neutralising with acid and,generally, diluting with water. The concentrates may be low in water orwater-free and generally neutralised. They are mixed with water in thecase of the preparation of a fresh CDL bath or, in the case of solidscompensation, with the contents of a CDL bath depleted of solids by CDLdeposition. The addition of sulfonic acid salt essential to theinvention may be carried out according to all the addition methodsdescribed above.

Regardless of whether the CDL coating compositions according to theinvention are pigmented or transparent, they may also be prepared, forexample, by the so-called two-component process. In that case, theaddition of sulfonic acid salt essential to the invention may be carriedout as described below.

The two-component CDL materials which can be used for producing the, CDLcoating compositions according to the invention are a) a bindercomponent in the form of a solvent-free or low-solvent, aqueousdispersion containing CDL binder and, optionally, crosslinking agents(CDL dispersion) and b) a separate pigment paste and/or catalyst paste.In the case of CDL coating compositions pigmented with opacifyingpigments, a pigment paste b) containing opacifying pigments is used. Inthe case of transparent CDL coating compositions, a pigment paste b)containing transparent pigments can be used. Component a) and/orcomponent b) many contain sulfonic acid salt.

The two-component CDL materials are preferably a) a binder component inthe form of a solvent-free or low-solvent, aqueous dispersion containingCDL binder and, optionally, crosslinking agent (CDL dispersion) and b1)a separate pigment paste and/or b2) a separate catalyst paste containingsulfonic acid salt or b3) a separate catalyst preparation containingsulfonic acid salt. In the case of CDL coating compositions pigmentedwith opacifying pigments, a pigment paste b1) containing opacifyingpigments is used. In the case of transparent CDL coating compositions, apigment paste b1) containing transparent pigments can be used. If apigment paste b1) is used, it may contain the sulfonic acid saltcatalyst, in which case a catalyst paste b2) or catalyst preparation b3)is not necessary. If the pigment paste b1) does not contain sulfonicacid salt catalyst, or if the CDL coating compositions are preparedwithout the use of a pigment paste b1), then a catalyst paste b2) or,preferably, a catalyst preparation b3) is used.

Dilution of components a) and b) or a) and b1) and/or b2) or b3) withwater or with the contents of a CDL bath depleted of solids by CDLdeposition is preferably carried out separately, for example by separatebut parallel addition to the water (preparation of a fresh CDL bath) orof CDL bath depleted of solids (solids compensation).

The pigment pastes b1) may contain the sulfonic acid salts or may befree thereof. Pigment pastes b1) can be prepared by dispersing thepigments and fillers and, optionally, the sulfonic acid salts in CDLbinders, preferably in CDL paste resins. Such resins are known to theperson skilled in the art. Examples of paste resins that can be used inCDL baths are described in EP-A-0 183 025 and in EP-A-0 469 497.

Catalyst pastes b2) can be prepared, for example, by pre-mixing, forexample pre-dispersing, suitable metallic or organometallic compoundswith sulfonic acid in the presence of organic solvents and/or water, andsubsequently dispersing and optionally milling the resulting mixturewith CDL binder, preferably with CDL paste resin and water.

The catalyst preparation b3) may be a finely divided suspension, forexample a colloidal or molecular solution; in the simplest case it is anaqueous solution of the metallic sulfonic acid salt or organometallicsulfonic acid salt.

From the CDL coating compositions according to the invention, CDLcoating layers can be cathodically deposited in the conventional manneron plastics substrates that are electrically conductive, for examplethat are electrically conductive or have been rendered electricallyconductive, for example that have been rendered electrically conductiveby metallisation, or, especially, on metal substrates. Accordingly, theinvention also provides a method of cathodically depositing CDL coatingcompositions according to the invention on such substrates.

There may be used as metal substrates parts made of all conventionalmetals, for example the metal parts conventionally employed in theautomotive industry, especially automotive bodies and parts thereof.Examples are components of aluminium, magnesium or alloys thereof, andespecially steel, for example ungalvanised or galvanised with pure zinc,zinc-nickel alloy or zinc-iron alloy. The metal substrates may bephosphated and passivated in the conventional manner. The corrosionprotection of primer coats deposited from the CDL coating compositionsaccording to the invention on, for example, steel is also outstanding onbright steel or on steel that has only been phosphated but notpassivated. The various substrates may be present jointly on a workpiece(mixed construction). It is also possible for metal parts or plasticsparts that have already been partially or completely pre-coated to bepresent on the workpiece, which parts pass through the method accordingto the invention, for example, unchanged, that is to say no CDL coatinglayer in particular is deposited on their surface in the course of themethod according to the invention.

After coating with the CDL coating composition according to theinvention, the coating is crosslinked by stoving, for example, indirectly and/or indirectly fired stoving ovens, for example at objecttemperatures of from 130 to 200° C. If the coating is, for example, aCDL primer coat, then further subsequent layers may be applied.

The CDL baths according to the invention are lead-free and exhibit no oronly extremely slight sedimentation phenomena even when subjected toshear.

EXAMPLE 1 Preparation of Bismuth Hydroxycarboxylic Acid Salts

Deionised water and hydroxycarboxylic acid are placed in a vessel andheated to 70° C. Commercially available bismuth oxide (Bi₂O₃) is addedin portions, with stirring. After a further 6 hours' stirring at 70° C.,the batch is cooled to approximately 20° C. and left for 12 hourswithout stirring. Finally, the precipitate is filtered off, washed witha small amount of water and ethanol and dried at a temperature of from40 to 60° C.

The following salts are prepared using the amounts indicated:

Bismuth Lactate:

466 parts (1 mol.) of bismuth oxide+901 parts (7 mol.) of lactic acid70% in water

Bismuth Dimethylolpropionate:

466 parts (1 mol.) of bismuth oxide+938 parts (7 mol.) ofdimethylolpropionic acid+2154 parts of water

EXAMPLE 2 Preparation of Bismuth Methanesulfonate

A mixture of 296 g of delonised water and 576 g (6 mol.) ofmethanesulfonic acid is placed in a vessel and heated to 80° C. 466 g (1mol.) of commercially available bismuth oxide (Bi₂O₃) are added inportions, with stirring. After 3 hours, a cloudy liquid forms which,when diluted with 5400 g of deionised water, yields an opalescentsolution. After concentration of the solution by evaporation, bismuthmethanesulfonate remains.

EXAMPLE 3 Preparation of CDL Dispersions

a) 832 parts of the monocarbonate of an epoxy resin based on bisphenol A(commercial product Epikote 828) are mixed with 830 parts of acommercially available polycaprolactone polyol (commercial product CAPA205) and 712 parts of diglycol dimethyl ether and reacted at from 70 to140° C. with approximately 0.3% BF₃ etherate until an epoxy number of 0is reached. To that product there are added at from 40 to 80° C., in thepresence of 0.31% Zn acetylacetonate as catalyst, 307 parts of areaction product of 174 parts of toluyne diisocyanate and 137 parts of2-ethylhexanol with addition of 0.3% benzyltrimethylammonium hydroxide.The reaction is carried out until the NCO value is approximately 0, andthe mixture is then adjusted to a solids content of 70 wt. % withdiglycol dimethyl ether.

b) To 1759 parts of a bicarbonate of an epoxy resin based on bisphenol A(commercial product Epikote 1001) there are slowly added at from 60 to80° C. 618 parts of a reaction product of 348 parts of toluenediisocyanate and 274 parts of 2-ethylhexanol with addition of 0.3%benzyltrimethylammonium hydroxide. The reaction is continued to an NCOvalue of approximately 0.

c) To 860 parts of bishexamethylenetriamine dissolved in 2315 parts ofmethoxypropanol there are added at a temperature of from 20 to 40° C.622 parts of the reaction product of 137 parts of 2-ethylhexanol with174 parts of toluene diisocyanate with benzyltrimethylammonium hydroxidecatalysis (0.3%), and the reaction is carried out to an NCO content ofapproximately 0. Then 4737 parts of the reaction product b) and 3246parts of the reaction product a) (each 70% in diglycol dimethyl ether)are added and the mixture is reacted at from 60 to 90° C. The reactionis terminated at an amine number of approximately 32 mg KOH/g. Theresulting product is distilled off in vacuo to a solids content ofapproximately 85%.

d1) Neutralisation is carried out with 30 mmol. of formic acid/100 g ofresin. The mixture is then heated to 70° C. and, in the course of twohours, bismuth lactate (from Example 1) is added in portions, withstirring, in such an amount that 1.5 wt. % of bismuth, based on thesolids content, are present in the batch. Stirring is then carried outfor a further 6 hours at from 60 to 70° C. After cooling, the batch isconverted with deionised water into a dispersion having a solids contentof 40 wt. %.

d2) The procedure described in d1) is followed, but bismuthmethanesulfonate (from Example 2) is used instead of bismuth lactate.

EXAMPLE 4 Preparation of a Pigment Paste

15 parts of acetic acid (50%), 30 parts of a commercially availablewetting agent (50%) and 374 parts of deionised water are added, under ahigh-speed stirrer, to 223 parts of the paste resin according to EP-A-0469 497 A1 Example 1 (55%).

5 parts of carbon black, 5 parts of pyrogenic silica and 560 parts oftitanium dioxide are added thereto. The mixture is adjusted to a solidscontent of approximately 50% with deionised water and milled in a beadmill. A stable pigment paste is formed.

EXAMPLE 5 Preparation of a Bismuth-containing CDL Bath, Comparison

4.5 parts of formic acid (50%) and 1760 parts of deionised water areadded to 815.5 parts of the dispersion of Example 3 d1). 420 parts ofpigment paste according to Example 4 are added with thorough stirring.The resulting CDL bath is poured through a sieve of mesh size 30 μmwhich has previously been weighed. After drying of the sieve, thesieving residue of the CDL bath is determined by re-weighing. It is lessthan 10 mg/l CDL bath. The CDL bath is subjected to shear by beingstirred for 48 hours, while covered, using a magnetic stirrer(Teflon-coated magnetic stirring rod). The sieving residue of the CDLbath is then determined again: 83 mg/l CDL bath.

EXAMPLE 6 Preparation of a Bismuth-containing CDL Bath, According to theInvention

4.5 parts of formic acid (50%) and 1760 parts of deionised water areadded to 815.5 parts of the dispersion of Example 3 d2). 420 parts ofpigment paste according to Example 4 are added, with thorough stirring.The sieving residue before and after exposure to shear is determined ina manner analogous to that of Example 5. It is less than 10 mg/l CDLbath both before and after exposure to shear.

What is claimed is:
 1. Aqueous, lead-free, cathodically depositablecoating compositions comprising binders of resin solids and, optionally,crosslinking agents, pigments, and conventional additives and whereinthe coating compositions comprise at least one alkanesulfonic acid saltselected from the group consisting of bismuth alkanesulfonic acid salts,organobismuth alkanesulfonic acid salts and any mixture thereof in atotal amount from 0.2 to 2 wt. %, calculated as bismuth metal and basedon the resin solids.
 2. The aqueous coating compositions according toclaim 1 in which the alkanesulfonic acid salts comprise a substitutedalkyl radical.
 3. The aqueous coating compositions according to claim 1wherein the alkanesulfonic acid salts consist of methanesulfonic acidsalts.
 4. Process for the production of an aqueous coating compositionaccording to claim 1 which comprises incorporating the sulfonic acidsalts into an aqueous dispersion of the binder of the coatingcomposition and processing the resulting dispersion of binder andsulfonic acid salts to form the aqueous coating composition.
 5. Processaccording to claim 4 wherein the sulfonic acid salts are incorporated inthe form of a pigment paste containing pigments and binder and compoundsselected from the group consisting of fillers, paste resin and anymixtures thereof.
 6. Process according to claim 4 wherein the sulfonicacid salts are incorporated in the form of a catalyst paste comprisingbinder or paste resin.
 7. Process according to claim 4 wherein thesulfonic acid salts are incorporated in the form of a catalystpreparation consisting of a suspension or solution.
 8. Method ofcathodic dipcoating a substrate having a conductive surface byconnecting the substrate as a cathode in an electrocoating bathcontaining the coating composition of claim 1 and cathodicallydepositing a coating of the composition of claim 1 on the substrate andremoving the substrate from the bath.
 9. The method according to claim 8wherein the substrate is a motor vehicle body or a motor vehicle part.10. A Substrate having a coating of the composition of claim 1.